Loss of the auditory sense as one of the senses of human body leads personally to disorders in daily life and socioeconomically to huge losses. Deafness not only induces hearing impairment, but also when a serious deafness breaks out before language acquisition, inhibits the normal language development to causes the problem of accompanying a speech impediment.
Patients suffering from moderate or more deafness, who are officially registered within the country as a hearing-impaired person, amount to about 150 thousands as of June, 2008 in Korea. Further, in view of the fact that the ratio of patients who were born with congenital, bilateral moderate deafness is 0.1%, the ratio of patients developing the acquired deafness is significantly higher. In addition, people with hearing and speech impairment amounts to 10% (Status of registered disabled people on 2002) of a total of registered disabled people in Korea, and thus, is a major cause of physical handicap. Loss of bilateral auditory sense is relevant to 100% loss rate of labor capacity according to McBride assessment of disablement and does not allow for a people to perform the normal economical activities, thereby resulting in an enormous socioeconomical loss. By way of example, it has been known that an astronomical amount of money would be required for a disabled people in the project, assuming an about 300-billion-won budget (Budget for disabled people as earmarked for disabled people by the Ministry of Health and Welfare of Korea for the year 2005—286.8-billion-won) supported by the government of Korea for assistance to a disabled people and the social indirect cost for a disabled people.
Further, with the geometrical rise in aged population, presbycusis as recently emphasized, together with hypertension and degenerative arthritis is one of three age-related diseases, and has been known as a disease having a high prevalence rate which is found in 25-40% of the number of people over sixty-five in Korea. According to the 2000′ data of the National Statistical Office of Korea, the number of people over sixty exceeds 5 millions, and thus the potential support ratio amounts to about 11%, which constitutes an object of public concern. In addition, a disease of sensory organs, such as auditory sense, of the elderly people is also an important element limiting social activities of elderly man. Thus, it is urgently required to study degenerative sensory diseases and to develop a therapeutic agent thereof.
Among drugs used for patients suffering from various diseases including cancers, many drugs exhibit the ototoxicity and can be represented by aminoglycoside-based antibiotics and anticancer agents such as cisplatin, etc. Their typical side-effects include nephrotoxicity and ototoxicity. As compared to the nephrotoxicity, the ototoxicity irreversibly leads to a bilateral permanent loss of hearing and vestibular function in many cases, and the prevalence rate of ototoxicity is high to reach 7.5% as it has been reported to be 0.6-30% for cochlea toxicity and 0-75% for vestibular toxicity depending on the reporters in Korea.
Meanwhile, in addition to presbycusis and ototoxic deafness the common cause of acquired deafness is noise deafness, which is recently on the rise. According to the data reported in Korea in the year 1995, among a total of 782,274 cases of the special health examination for workers under harmful environments the cases of special examination for noise was the largest as 364,244 cases (44.3%), and among a total of 5,942 cases suffering from occupational diseases developed in the year 1992 the number of patients diagnosed as noise deafness was 3,345 beyond the half thereof. In addition, according to the result of health examination practiced in Korea in the year 1999, among a total of 2,713,240 workers from 115,761 working places 1,794 cases were confirmatively diagnosed as an occupational disease, wherein 1,056 cases to be 58.9% thereof were identified as noise deafness. Thus, it can be noted that among the occupational diseases a ratio of noise deafness is very high. Therefore, the noise has emerged as the problem which should be solved with having an overriding concern.
It is considered that if the mechanism of apoptosis of auditory organs due to various causes (drugs, radiation, noise, environmental pollution—electromagnetic wave, etc.) would be revealed and a material and gene, which can control of such apoptosis of auditory organs would be found, deafness originated from various causes or unknown cause will be prevented or treated. Recently, as the study of inner ear hair cell damage related to deafness has been actively progressed, a death of auditory hair cell due to free oxygen radical (reactive oxygen species, ROS) has been reported as the main cause of deafness, and the search and development of an auditory protective agent related thereto is being actively made.